Wireless communication devices with radio frequency (RF) integrated circuits (RFICs) may incorporate radio frequency voltage-controlled oscillators (RF VCOs) which are used as local oscillators to convert baseband communication channels to and from one of many RF channels.
RF channel tuning is accomplished by phase-frequency locking a RF VCO output frequency signal with a reference frequency, typically derived from a reference oscillator, in conjunction with a RF phase-locked loop (RF PLL). In wireless communication devices, the RF PLL is usually part of the same RFIC as the RF VCO. The RF PLL compares the RF VCO output frequency (utilizing a frequency divider) with the reference frequency. The RF PLL output provides a correction signal derived from the phase-frequency difference between the reference frequency and the frequency divided RF VCO output frequency.
The correction signal is, in turn, filtered (using a loop filter) to produce an analog control voltage for input to the RF VCO, and serves as a fine frequency tuning signal. When the RF VCO is not in phase or frequency lock with the reference frequency, the fine frequency tuning signal (the filtered correction signal) converges to a value (either increases or decreases in voltage) until the RF VCO output frequency is phase and frequency locked to the reference frequency. If the RF VCO cannot maintain phase and frequency lock, the wireless communication link performance, as measured at the RF channel, will not function properly or not at all.
Wireless communication devices operating across multiple radio frequency bands benefit from a RF VCO having wide frequency tuning range. Wide frequency tuning range is achieved with multiple tuning elements (comprised of fine and coarse tuning elements) in the RF VCO. Fine frequency tuning is provided by the fine tuning elements, while coarse frequency tuning is provided by the coarse tuning elements. During fine frequency tuning, the RF PLL and loop filter provide a fine frequency tuning signal to the fine tuning element within the RF VCO. Coarse frequency tuning is accomplished by switching in or out various discrete coarse tuning elements (setting a coarse frequency tuning code) to shift the RF VCO output frequency in large steps.
Unfortunately, the fine and coarse tuning element component values vary significantly with changes in operating temperature and operating voltage, leading to frequency drift in the RF VCO for a given fine frequency tuning signal voltage and coarse frequency tuning code. This frequency drift must be compensated for to ensure that the RFIC, along with RF VCO and RF PLL, properly tunes to the desired RF channel. In extreme cases, the frequency drift may exceed the fine frequency tuning signal voltage capability of the RF PLL and the RF VCO if the coarse frequency tuning code is held constant for a specific RF channel.
Coarse frequency tuning, in combination with fine frequency tuning, also allows the RFIC to better compensate for IC process variations. Coarse frequency tuning may be utilized as a method for reducing IC process variations affecting the RF VCO output frequency vs. coarse frequency tuning code (coarse frequency calibration).
Coarse frequency calibration may be done by frequency locking the RF PLL (with the fine frequency tuning signal) and RF VCO (with both fine frequency tuning signal and coarse frequency tuning code inputs) across multiple operating frequencies at circuit startup to compensate for process variations at a starting operating temperature. The final step of coarse frequency calibration is to store coarse frequency tuning codes across a range of desired RF VCO output frequencies.
Alternatively, coarse frequency calibration may be done only once, usually when the RFIC, including the RF VCO, is installed in a wireless communication device and is ready to be programmed and tested in a factory. In this case, the coarse frequency calibration is completed when the coarse frequency tuning codes are stored during factory testing over a range of desired RF VCO output frequencies at a nominal factory operating temperature. A third method may perform coarse frequency calibration at both circuit startup and in a factory environment.
As mentioned above, coarse frequency calibration may be done on multiple RF channels and/or operating RF bands (cellular, PCS, GPS, UMTS, GSM, etc) and multiple RF VCOs (transmit, receive, GPS, Bluetooth, etc). Coarse frequency tuning codes are generated during calibration and stored in the wireless communication device memory for later use as coarse frequency tuning during operation of the device with particular frequency bands or operating channels.
Conventional fine and coarse frequency tuning calibration techniques suffer in certain circumstances. In one instance, the RF VCO coarse frequency calibration is only performed at the beginning of the wireless communication device operation (on power-up), and at an initial temperature which changes after the coarse frequency calibration is complete.
One of the worst case conditions is to perform fine and coarse frequency tuning calibration at the coldest operating temperature (often below 0 C., freezing) and observe the RF VCO circuit behavior as the wireless communication device temperature rises from self-heating during normal operation. In the event that the coarse frequency tuning code is not changed for a given RF IC operating frequency, the fine frequency tuning signal (or voltage) is observed falling outside operating (voltage) limits. In this situation, the RF PLL will not be able to maintain frequency lock. Alternatively, the phase noise of the RF VCO may be significantly compromised as might occur when the fine frequency tuning signal approaches its operating (voltage) limit for a given coarse frequency tuning code. As explained above, coarse frequency tuning codes are matched to a particular RF operating channel during calibration and kept constant post-calibration. In either scenario, the wireless communication device may fail critical performance tests as it lacks proper calibration.
Given the limitations of RFICs utilizing wide-band RF VCOs with the requirement for coarse frequency tuning codes and fine frequency tuning signals, a more optimal design to deal with RF VCO frequency tuning variations for operating temperature changes is desirable.